Composites comprising a support or substrate onto which a layer of diamond or of diamond-like carbon is deposited have been known for a long time.
Such composites are customarily obtained by chemical vapor deposition (CVD) of carbon atoms onto the substrate. The methods for carrying out the CVD process for obtaining diamond coatings are described in B. V. Spitsyn et al., J. of Crystal Growth 52 (1981), 219-26 and S. Matsumoto et al., J. of Materials Science 17 (1982), 3106-12. The document by J.-P. Manaud et al., Surface & Coatings Technology 202 (2007), 222-231 describes the formation of diamond seeds obtained without polarization of the substrate which resulted in the formation of cubo-octahedral microcrystals. The size of these crystals varies from a few microns to a few tens of microns. However, these composites are not without drawbacks: the diamond is composed of covalent tetrahedral carbon with very strong sp3 hydridized bonding. Therefore, the nuclei have little aptitude for creating bonds of a chemical nature with the substrates. The internal tensions of the coating, the poor adhesion of the coating to the substrate and the difference in the thermomechanical properties between the coating and the substrate result in high brittleness of these composites.
However, these composites are designed to be used in very sensitive sectors such as the manufacture of tools for machining composites intended for the aeronautics industry, and these composites are highly abrasive and destructive for the cutting tool. They may also be used as implants in the biomedical field, or else they may be used as tools in odontology. Their use has also been proposed in the field of chemical engineering as mechanical seals for pumping corrosive liquids due to the high chemical inertness of the diamond.
In all these applications, a high reliability of the diamond materials is expected, but the processes currently known do not make it possible to obtain the desired result.
In particular, as regards the manufacture of tools for machining composites, these are extremely abrasive so that the cutting edges of the tools are eroded and rounded off from the first contact with the material. Local heating ensues, which accelerates the destruction of the tool and damages the machined surface (local melting, delamination, etc.). The protection of the surface of the cutting tools is therefore essential. Diamond is a material of choice since it is reputed to be the hardest present in nature. In recent years, tools treated with MCD (microcrystalline diamond) and more recently NCD (nanocrystalline diamond) have appeared. The first treatment was very rapidly characterized by its lack of reliability. The second, more performing, did not achieve however the properties of PCD (polycrystalline diamond: coated diamond crystals) tools. Unfortunately, the manufacture of the latter, for certain types of tools such as drills or small-diameter cutters remains to date nonexistent because of being technically difficult and economically very costly.
And in particular, in the case of tools based on cobalt tungsten carbide (WC—Co), which are used for machining parts made of highly abrasive materials such as nonferrous metal alloys, composites and ceramics, the presence of cobalt in the substrate (generally between 6 and 10%) has a negative influence on the diamond deposition process. The presence of Co promotes the formation of non-diamond carbon-based phases, that is to say for the most of graphite, which leads to a weak adhesion of the diamond-like coating to the substrate.
To solve this problem, various methods have been proposed, for improving the quality of the diamond coating and its adhesion to the substrate: chemical pickling, which depletes the substrate of cobalt at its surface, and the deposition of a diffusion barrier. However, none of these methods is sufficient to completely solve the problem.
The reaction scheme for the deposition of a diamond-like coating comprises three steps: initiation, nucleation and growth.
In the processes customarily used, in the nucleation step, the supersaturation in carbon (G. Cicala et al., Diamond & Related Materials 14 (2005), 421-425; S. J. Askari et al., Vacuum 81 (2007), 713-717; S. J. Askari et al., Diamond & Related Materials 17 (2008), 294-299) in the absence of an electric field results in an increase in the presence of graphite (sp2) species and therefore a reduction in the adhesion of diamond (sp3) nuclei (F. A. Almeida et al., Vacuum 81 (2007), 1443-1447).
Certain authors (H. Sein et al., Diamond & Related Materials 13 (2004), 610-615; H. Li et al., Diamond & Related Materials 16 (2007), 1918-1923) have proposed using a supersaturation in carbon together with an electric field in the nucleation step, so as to obtain a very high nucleation density, an increase in the adhesion of the seeds and therefore a better adhesion of the coating. But these properties must be improved further in order to satisfy the requirements linked to applications in materials having a diamond coating.
Documents CN 1827846 and JP 19920071435 describe a process for growing diamond films. This process comprises a nucleation step, under an electric field of negative voltage and a deposition or growth step under an electric field of positive voltage. This process results in the formation of micro-diamond grains, the morphology and texturing of which are very similar to those obtained by growth in the absence of an electric field.